Fan Control Circuit

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A fan is usually not needed to run at full speed all the time. If it is still cool enough inside the case, there is no need for fans. They just create noise. If you want to have the fans running only when needed, you need some kind of temperature dependent control.

The easiest would be a NTC resistor in series to the fan. This method has some disadvantages. The NTC has to be calculated specific to the fan, the resistance of the NTC will drop because of the current to the fan passing through it. If the resistance of the NTC is too high, the fan would not start when it is really cold. The NTC will have a series resistance and keeps the fan from running full speed, even when hot.

The more advanced method would be a comparator circuit. Inspired by this Ars Technica article, I've built the following:

Here is the schematic of the circuit.

The circuit alleviates all the problems mentioned above:

It's fairly independent of the fan used. It will work with fans in the 0.5 to 5 W area.

It doesn't run much current through the NTC, thus it won't drift away.

The ZD5.1 provides a minimum voltage to the fan, even if it is freezing cold.

The 47 µF capacitor gives full supply voltage to the fan at power-on, to get the fan started.

The maximum voltage to the fan would be almost the supply voltage.

And last, but not least, it can be set to almost any temperature range.

Interested? OK, here is the component list:

10k NTC
3.9 Resistor
1.8k Resistor
15k Resistor
5.1 Zenerdiode
47µ Capacitor
100µ Capacitor
10k Potentiometer
50k Potentiometer
TL081 Operation amplifier
BUZ20 MOS-Transistor

Those things should be available for around 2€ in electronic stores.

How it is set to work:

Start with both potentiometers around their middle setting.
With the NTC at room temperature, set the 10k poti to the desired minimum voltage.
Heat the NTC up to the temperature, at which you want the fan running at full speed.
Set the 50k poti so, that the full voltage goes to the fan.
Let the NTC cool back to room temperature.
Repeat steps 2...4 if necessary.

You wont need a voltage meter, but it would help tremendously.

I will update this page, as soon as I get a dedicated board for it.

I might also write a functional description, if I get into the mood ...

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For those, who save pages to disk, but notoriously lose URLs, this page is http://people.freenet.de/s.urfer/fan_control.htm